sql2

select count(列名) from 表名 #查询列中不同字段数

 

 distinct  去重

where 条件查询 在group by 之前

hving 在 group by 之后

 

如果在from 后面的表必须得起别名

在 where 后面的表不需要

 

 

SQL 语句的书写顺序如下：

SELECT -> FROM -> WHERE -> GROUP BY -> HAVING -> ORDER BY

SQL 语句的执行顺序如下：

FROM -> WHERE -> GROUP BY -> HAVING -> SELECT -> ORDER BY

 

 

select语句的处理顺序

1. FROM
2. ON
3. JOIN
4. WHERE
5. GROUP BY
6. WITH CUBE 或 WITH ROLLUP
7. HAVING
8. SELECT
9. DISTINCT
10. ORDER BY
11. TOP

 

 

 

 

 mod(id, 2) = 1  #条件判断取模运算
order by  rating Desc  #按rating值排列 desc 反向

update Salary SET sex = if(sex = 'm','f','m') #1. update 表名　set 条件判断语句 # 2. if判断 if(列对象 = 值,case1,case2) 若为真,则执行case1,否则执行case2